1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to an apparatus and method for reporting the result of frame reception in a CDMA (Code Division Multiple Access) mobile communication system.
2. Description of the Related Art
Reporting the result of frame reception is a process where a receiver determines the reception state of a frame and transmits the determined state to a transmitter. The reception state may represent a CRC check result, the energy level of the received frame, or a different kind of reception quality. Based on the frame reception result, the transmitter performs power control for the receiver.
A receiver in a conventional mobile communication system, however, reports the reception result of a frame on one channel only to a transmitter. Therefore, when frames are received on two or more channels simultaneously, a report is made on only one of the channels.
The conventional reporting of a frame reception result will be described in more detail in conjunction with a CDMA mobile communication system standardized in TIA TR45.5/3GPP2 (referred to as a CDMA 2000 system). It is assumed that the transmitter is in a base station and the receiver in a mobile station. The CDMA 2000 system uses a fundamental channel (FCH), a dedicated control channel (DCCH), a supplemental channel (SCH), and a supplemental code channel (SCCH) as traffic channels.
Two cases may be considered in describing the reporting of a frame reception result: using one traffic channel as shown in FIG. 1A and using at least two traffic channels as shown in FIG. 1B.
Referring to FIG. 1A, a base station transmitter 110a transmits a frame on one of the traffic channels to a mobile station receiver 120a and the mobile station receiver 120a reports the reception result of the frame back to the base station 110a. In this case, frame reception and reporting of the reception result occur only on one channel, thereby avoiding the problem of excluding other traffic channels from reporting reception results. It is clear, however, that such an operation cannot provide services on a plurality of traffic channels.
Referring to FIG. 1B, a transmitter 110b transmits frames on at least two traffic channels, that is, a first traffic channel frame and a second traffic channel frame to a receiver 120b and the receiver 120b reports a reception result back to the transmitter 110b. In this case, while the first and second traffic channel's frames are received, only the reception result of the first traffic channel frame is reported to the transmitter 110b, excluding the second traffic channel frame from the reporting operation. The first traffic channel can be the FCH or the DCCH and the second traffic channel can be the SCH or the SCCH.
FIG. 2 illustrates the temporal relationship between a frame received on a traffic channel and a transmit frame having the reception result of the received frame in the conventional technology. Referring to FIG. 2, if the receiver receives an (i+1)th frame on the traffic channel, it transmits the reception result of the (i+1)th frame in an (i+3)p frame after two frames. The two frame-delay happens because frames are transmitted and received in synchronization at the mobile station and thus the reception result of the (i+1)th receive frame cannot be transmitted in an (i+1)th transmit frame. In addition, the time required to process the (i+1)th received frame makes it impossible to transmit the reception result of the (i+1)th received frame in an (i+2)th transmit frame. In FIG. 2, one frame is 20 ms in duration.
FIG. 3A illustrates a multiplexer (MUX) for multiplexing frame reception result indicator bits and a multiplexing method in the receiver operated as in FIGS. 1A and 1B. In FIG. 3A, a pilot and a power control bit (PCB) are multiplexed in time to a reverse pilot channel in the CDMA 2000 system. FIG. 3B illustrates the structure of a power control group (PCG). One 20-ms frame includes 16 PCGs, each PCG having a pilot and a PCB. The PCB can be replaced by a reception result indicator bit. Each PCB has a unique value in a corresponding PCG, enabling a fast power control at a rate of up to 800 Hz, the reception result indicator bit has a fixed value in one frame. Therefore, the base station transmitter performs a slow power control at 50 Hz using the reception result indicator bit. Hereinafter, a bit indicative of power control information or reception result information such as the PCB or the reception result indicator bit will be referred to as a “control bit”. In the CDMA 2000 system, a control bit occurs 16 times in one 20-ms frame because one 20-ms frame includes 16 1.25-ms PCGs, each PCG having a unique control bit. One 1.25-ms PCG is divided into four 0.3125-ms groups, each having 384N chips. The last of the four groups is reserved for the control bit. Let a spreading rate of 1.2288 Mcps be 1. Then, 384 means the number of chips in one group and N is a spreading rate. For example, if a spreading rate is 3, that is, 3.6864 Mcps, 384×3 chips are assigned to each group by
                              chip          ⁢                                          ⁢          number                =                  1.25          ⁢                                          ⁢          ms          ×                                    1.2288              ⁢                                                          ⁢              Mcps                                      1              ⁢                                                          ⁢              sec                                ×          N                                    (        1        )            
When frame reception result indicator bits are used as the control bits in the conventional technology, 16 power control bits in one frame are set to represent one reception result value. In other words, the reception result of a frame on one traffic channel is set in 16 PCBs.
FIG. 4A illustrates the structure of a frame containing a reception result reported by the conventional mobile station. In FIG. 4A, all frame reception result indicator bits in one frame are set to an identical value, that is, they represent the reception result of one received frame.
Referring back to FIG. 1B, the base station 110b transmits the first and second traffic channels to the mobile station 120b but the mobile station 120b reports the reception result of only one channel, i.e., the first traffic channel in the frame shown in FIG. 4.
The reception result may be used for the base station to control the transmission power of the mobile station. If the reception result has been reported as good, the base station instructs the mobile station to gradually reduce its transmission power and vice versa, so that the mobile station can receive a signal at an acceptable quality level. This is a slow power control on a frame basis that may be implemented only if a fast power control is not feasible. The fast power control occurs 16 times per frame. The first traffic channel may differ from the second traffic channel in data rate, code rate, and QoS (Quality of Service) requirements. This implies that it is necessary to implement power control on the traffic channels separately because of the difference in characteristics between the traffic channels.
However, when reporting a reception result is limited to one channel as in the conventional technology, it is impossible to control the transmission powers of the other channels.
Meanwhile, a fast power control using PCBs may be used for power control of two traffic channels. That is, the mobile station transmitter transmits the PCBs for the fast power control on each of the two traffic channels. However, fast power control is not reliably done if an SCH is transmitted in a DTX (Discontinuous Transmission) mode or at a variable rate. In the DTX mode, an outer loop set point is not maintained appropriately, and at a variable rate, the requirement of rate detection before PCB transmission is not satisfied.